A fuel cell is a cell for taking out an electric energy which is obtained by directly converting a chemical energy of a fuel by oxidizing hydrogen or methanol or the like. The fuel cell is attracting attention as a clean electric energy supply source. In particular, since a solid polymer electrolyte-based fuel cell works at a lower temperature as compared with others, it is expected as an automobile alternative power source, a domestic cogeneration system, a portable generator or the like.
The solid polymer electrolyte-based fuel cell comprises at least a membrane electrode assembly in which a gas diffusion electrode obtained by laminating an electrode catalyst layer and a gas diffusion layer is joined on each of both surfaces of a proton exchange membrane. The term “proton exchange membrane” as used herein refers to a material having a strongly acidic group such as a sulfonic acid group and a carboxylic acid group in the polymer chain and having a property of selectively passing a proton. As the proton exchange membrane, there is suitably used a perfluoro-based proton exchange membrane represented by Nafion (registered trademark, produced by Du Pont) having high chemical stability.
During the operation of a fuel cell, a fuel (for example, hydrogen) is supplied to the gas diffusion electrode on the anode side, an oxidant (for example, oxygen or air) is supplied to the gas diffusion electrode on the cathode side, and both electrodes are connected through an external circuit, thereby activating the fuel cell. Specifically, when hydrogen is used as the fuel, the hydrogen is oxidized on an anode catalyst to produce a proton. This proton passes through a proton conductive polymer in the anode catalyst layer, then moves in the proton exchange membrane and passes through a proton conductive polymer in the cathode catalyst layer to reach on a cathode catalyst. On the other hand, an electron produced simultaneously with the proton by the oxidation of hydrogen passes through the external circuit to reach the gas diffusion electrode on the cathode side. On the cathode catalyst, the proton and the oxygen in the oxidant react to produce water, and an electric energy is taken out at this time.
In this case, the proton exchange membrane is also required to act as a gas barrier wall. If the proton exchange membrane has a high gas permeability, the hydrogen on the anode side leaks toward the cathode side and the oxygen on the cathode side leaks toward the anode side, that is, cross leak occurs, thereby producing a so-called chemical short state and unabling to take out a good voltage.
The solid polymer electrolyte-based fuel cell is usually operated in the vicinity of 80° C. in order to obtain a high output property. However, when the fuel cell is used for vehicle applications, assuming the vehicle travel in the summer season, the fuel cell is required to be operable even under high-temperature low-humidification conditions (an operation temperature in the vicinity of 100° C. with 50° C. humidification (corresponding to a humidity of 12 RH %)). However, if a fuel cell is operated using a conventional perfluoro-based proton exchange membrane for a long time under high-temperature low-humidification conditions, there occurred a problem that pinholes are produced in the proton exchange membrane and cross leak is caused. That is, a sufficiently high durability is not obtained by the conventional perfluoro-based proton exchange membrane.
As the method for improving the durability of the perfluoro-based proton exchange membrane, there are disclosed a method for improving the durability by reinforcement using a fibrillated polytetrafluoroethylene (PTFE) (Patent Documents 1 and 2), a method for improving the durability by reinforcement using a PTFE porous membrane subjected to stretch treatment (Patent Document 3) and a method for improving the durability by reinforcement by adding inorganic particles (Patent Documents 4, 5 and 6).
In addition, Patent Document 7 discloses a blended membrane of a perfluorosulfonic acid polymer and a polybenzimidazole and a method for improving chemical stability.
Further, Patent Document 8 discloses a proton exchange membrane containing polyphenylene sulfide particles.    Patent Document 1: Japanese Patent Laid-Open No. 53-149881    Patent Document 2: Japanese Patent Publication No. 63-61337    Patent Document 3: Japanese Patent Laid-Open No. 8-162132    Patent Document 4: Japanese Patent Laid-Open No. 6-111827    Patent Document 5: Japanese Patent Laid-Open No. 9-219206    Patent Document 6: U.S. Pat. No. 5,523,181    Patent Document 7: International Publication No. 2005/000949    Patent Document 8: International Publication No. 2005/103161